As semiconductor devices are scaled down to smaller and smaller dimensions, the ability to laterally restrict or confine the motion of the charge carriers in the devices to very small dimensions becomes increasingly important.
The planar confinement of excitons in semiconductor superlattices and quantum wells has resulted in linear and nonlinear optical properties that are greatly different from those of the constituent bulk materials. These new optical properties have been extremely useful in the development of novel optical devices. The possibility of confining excitons laterally--to "wires" or "dots" in the plane of the layered materials--as well as perpendicular to the layers is an intriguing one. Predictions of novel linear and nonlinear optical effects of confinement of carriers in all three dimensions have been made recently.
However, there has been relatively little experimental work reported on optical properties of laterally confined carriers in semiconductor superlattices and quantum wells. There are several recent reports of measurements of photo-luminescence and cathodoluminescence of "quantum dots", defined either by etching confining structures or by patterned ion implantation and annealing. Blue shifts of the exciton luminescence peak have been attributed to increased zero-point energy associated with the lateral confinement in some of these structures. Multiple peaks have been ascribed to the lifting of energy level degeneracies by lateral confinement.
There are both advantages and drawbacks to the fabrication methods reported to date for achieving lateral confinement of excitons in quantum wells. Confining excitons by etching through a well exposes a free surface which may be considerably damaged in the etching process. Patterned ion implantations with subsequent annealing results in patterned layer intermixing and a parabolic-like lateral potential well for carriers. This method for microstructuring is reported to yield large confinement energies. Its disadvantages, however, include its incompatibility with low temperature processing, limited control of the profile of the potential well, and possible non-recoverable damage to the electrical or optical properties of the material caused by the ion implantation process.
More particularly, B. G. Yacobi et al. have reported in two articles in Applied Physics Letters 52, 555-557 and 1806-1808 (1988) cathodoluminescence observations of metallization-induced stress variations in GaAs/AlGaAs multiple quantum well structures. The stresses are induced under 0.4 nm gold layers on the semiconductor structure and cause a change in the band gap of the material leading to both electron and hole confinement.
K. Yamonouchi et al. have reported in IEEE Journal of Quantum Electronics QE16 (6), 628-634 (1980) optical surface wave mode converters and modulators utilizing static strain-optic effects. Here, periodic perturbations of dielectric waveguides are obtained via the static strain-optic effect by evaporating an SiO.sub.2 thin film grating on a Ti diffused LiNbO.sub.2 waveguide. Strain appears in the waveguide when the thermal expansion coefficients of the evaporated film are different from those of the waveguide, owing to the temperature difference between the evaporation and operation of the device.
J. P. Wolfe et al. review in pages 433-437 in Electron-Hole Droplets in Semiconductors edited by C. D. Jeffries and L. V. Keldysh, North-Holland (1983) their work on recombination luminescence emanating from a strain-confined drop of electron-hole liquid in Ge and Si indirect bandgap semiconductors.
U.S. Pat. No. 4,683,484 issued to G. E. Derkits, Jr. on July 28, 1987 discloses non-invasive structures for laterally confining a single type of charge carrier in the narrow bandgap layers of a multiple quantum well semiconductor device useful in charge coupled devices. Here, confinement is attained by the formation of electric fields in a structure comprising alternating wide and narrow bandgap semiconductor layers to create steps in the conduction and/or valence bands.